288 THE INFLUENCE OF INANIMATE SUKKOnNDINGtS. 



formation of a new species in that spot. But the difficulty 

 of transport across the sea must, on the contrary, probably 

 prevent the frequent importation of new individuals of the 

 parent form, and consequently the formation of a new species 

 will be facilitated by the impediment thus offered to free cross- 

 ing with the original form. 



These inferences from the view that constant currents con- 

 stitute an important auxiliary in the diffusion of land moUusca 

 are, as every conchologist knows, in perfect accordance with the 

 facts. Most of the small species and of the operculated species 

 have a much wider range than the large inoperculated forms. 

 While the typical Philippine genera Goohlostyla, Rhyaota, 

 Ilelicarion, Ghlorcea, and Ohhina, which principally dwell in 

 trees, are found only, or almost only, in the Philippines, the 

 small genera, as Subulina, Trochomorpha, and Ennea, among the 

 Helicidse, and the Operculata Cyclophorus, Alycceus, Helicina, 



Fig. 15,-~-TrochomoriilM eq. 



and Diplommatina, have a very wide distribution, and at the 

 same time a remarkable uniformity of species. Thus Unnea 

 bioolor ranges from India to the Pacific Ocean, where I myself 

 found it in the Pelew Islands; the same species of Helicina, 

 Pupina, and Leptopoma occur in almost all the islands of 

 the Philippine Archipelago, while, notwithstanding the great 

 number of species in the genus Cochlostyla, no two identical 

 forms are to be found in Mindanao and Luzon. The species of 

 the genus Trochomorpha are extremely similar in appearance, 

 whether they come from India, the Moluccas, the Philippines, 

 or the islands of the Pacific ; nay, several species of this genus 

 are distributed throughout this vast region, almost without any 

 variation in their shells. 



In the closest connection with these facts is the theory, 

 which, under the name of the Migration theory— or, as it is now 

 called, the Separation theory — has been propounded by its origi- 

 nator Moritz Wagner in opposition to Darwin's theory of seleo- 



